In the art of earth-orbiting satellites it is known to establish a plurality of such satellites operating together as a system in order to carry out one or more system tasks, such as communication, mapping, or position determination. Frequently it is necessary that particular operations of each satellite comprising such a system be keyed to a precise time reference which should be uniform across the entire system. This requirement has generally been met by the placement of a highly accurate and stable clock in each such satellite. To guard against failure of such satellite clocks, each satellite has also been provided with one or more redundant standby clocks which may be activated in the event of such a failure. The placement of at least two such highly accurate and stable clocks in each satellite comprising such a system involves a very substantial expense to the system operator and a significant weight penalty which is manifested in higher launch costs. The present invention provides a system and method for maintaining synchronization of on-board clocks in such a system of orbiting satellites at a previously unrealized level of stability and accuracy and without the necessity for the redundant clocks used in prior art systems. The system and method of the invention further includes an optional elimination altogether of the highly accurate and stable clocks, and the attendant expense and weight associated with such clocks, from a portion of the satellites comprising such a satellite system.
At this point it is noted that a preferred embodiment of the invention is an application thereof in the Global Positioning System (GPS). Accordingly, to better illustrate the operation of the invention, the invention will be described in the context of a GPS application. As a predicate, the operation and characteristics of the GPS will be briefly described.
The GPS is a navigational system that makes use of state of the art satellite technology to provide users with a valuable tool for determining position. The system allows a navigator to rapidly determine his position by means of a small "GPS receiver", regardless of the navigator's position on the earth, and with a great deal of accuracy (usually within a few hundred feet. Such a system possesses many advantages that may benefit both civilian and military users. In the civilian setting, for example, a lost motorist who is equipped with a GPS receiver could pinpoint his position and take corrective measures. The system offers even greater benefits for the captain of a ship. Since the captain has fewer alternative methods of navigation than the motorist, he is more dependant on the information provided by his GPS receiver. In the military setting, the implications of the GPS are more vividly demonstrated. For example, the captain of a submarine carrying ballistic missiles can use GPS information to quickly obtain and accurately determine his submarine's latitude and longitude. Once the submarine's position has been accurately determined the trajectory to a target of known position can be computed and a missile can be deployed. The foregoing examples illustrate the value of the GPS and demonstrate how those who rely on the system must depend on its accuracy and reliability.
The Global Positioning System is composed of a plurality of satellites orbiting at approximately 11,000 nautical miles above the earth and maintained in almost perfectly circular orbits. These orbits are chosen so that the system can provide information to a user regardless of the time that the user requests information and regardless of the user's position on the earth's surface. Four of the orbiting satellites must be "visible" to the user at any one time in order for a position determination to be made. The satellites continuously broadcast their trajectory, clock offsets and radio-ranging signal. Once the GPS receiver has the range to each of four satellites, and the position of those four satellites, a determination of the receiver's position may be made.
A GPS receiver determines position by employing a three-dimensional equivalent of the traditional "triangulation" technique. Triangulation is the navigational technique whereby a platform on the earth's surface may compute its latitude and longitude by using only its range relative to two reference points of known position. In the GPS scenario, the platform may compute its latitude, longitude, and, if needed, its altitude by using its range relative to three satellites of known position. The additional coordinate of altitude may be required by those platforms not constrained to operation on the earth's surface. In theory, the three coordinates describing the user's position can be determined from three range measurements since the three measurements will yield three equations, three equations being sufficient to solve for three unknowns. However, in practice, the user clock always differs from the system clock, introducing a fourth unknown, namely the system time. Thus a practical implementation of the GPS position finding function requires that four range measurements be made. The range of the GPS receiver to four different satellites is computed so that four equations are then available for establishing a fix. From these four equations it is possible to determine the user clock offset as well as the three unknown coordinates.
A method which may be used to compute the range to a satellite involves the transmission by each satellite of an encoded pulse of electromagnetic energy. The pulse will be incident upon the receiver after a delay that is proportional to the distance from the satellite to the receiver. The pulse is then decoded by the receiver to determine the identity of the transmitting satellite, the time of transmission, and the position of the satellite at the time of transmission. When four such pulses are simultaneously transmitted, one by each of four different satellites, the receiver can calculate its range to each of the four satellites at the time of transmission and, from those ranges and the known positions of the satellites, may then calculate its position relative to the earth.
One of the larger sources of error in the receiver's position calculation is the variation in the time standards on board each satellite. A variation in the satellite time standards will cause the actual transmission times of the satellites to vary since each satellite keys its transmission from its own internal clock. A variation in the actual transmission times means that the measured ranges of the satellites will change between actual transmissions. Thus, the range of the satellite reference coordinates from which the GPS receiver calculates its position will not be accurately known. The inaccuracy in the range satellite makes an error free determination of the receiver's position impossible.
To reduce the errors caused by clock variation, the GPS satellites are equipped with atomic clocks which maintain highly accurate time standards based on atomic frequencies. Currently, each GPS satellite contains three such atomic clocks. As previously indicated, multiple atomic clocks are employed on each satellite to increase the operating life of the system by providing backup in the event that some of the clocks fail. These atomic clocks are very expensive and contribute significantly to the overall cost of the system. Accordingly, a timing method that would achieve a reduction in the number of atomic clocks required, without an accompanying reduction in performance, would greatly increase the cost efficiency of the GPS.